1. Field of the Invention
The embodiments discussed herein are related to a power diode used in a power converter and the like, a semiconductor device that has the power diode incorporated therein, and a method of manufacturing a semiconductor device.
2. Description of the Related Art
A freewheeling diode is a semiconductor device used in high-voltage and large-current power converting equipment and the like. Reduction of switching loss and soft recovery characteristics are requisite electrical characteristics at the time of switching of a freewheeling diode. Recently, soft recovery characteristics have been particularly demanded as measures for environmental problems to suppress electromagnetic noise arising from power electronic devices.
FIG. 5 is a cross-sectional view of relevant parts of a semiconductor substrate and depicts a layer structure of a conventional diode and a carrier lifetime control region. As depicted in FIG. 5, a vertical power diode 100 used as a freewheeling diode includes an anode electrode 102 provided on a top surface of an n-type drift layer 101 of high resistivity, and a cathode electrode 103 provided on a bottom surface of the n-type drift layer 101.
The anode electrode 102 forms an ohmic contact with a p-type anode layer 104 formed selectively at a central portion of a top surface side of the n-type drift layer 101. The cathode electrode 103 forms an ohmic contact with an n-type cathode layer 105 formed on the entire surface of a bottom surface side the n-type drift layer 101. The anode layer 104 that the anode electrode 102 contacts is a region associated with a principal current and is referred to as “active portion 109”.
An edge termination structure 110 is disposed at an outer periphery of the n-type drift layer 101, surrounding the anode layer 104, on the top surface side of the n-type drift layer 101, i.e., the same side where the anode electrode 102 and the anode layer 104 are disposed. The edge termination structure 110 includes guard rings 107 and field plates (not depicted). When reverse voltage is applied with the anode as a negative electrode, the guard rings 107 have a function of reducing a high electrical field generated at a surface of the substrate at the periphery of a pn junction 106. The field plate has, for example, a function of preventing a change of electrostatic potential induced by external charges.
The edge termination structure 110 has an insulating film 108 in addition to the guard rings 107 and the field plates. The insulating film 108 protects an edge termination surface 106a of the pn junction and a high-field silicon (Si) substrate surface on a side peripheral to the edge termination surface 106a. In the edge termination structure 110, a carrier lifetime control region 111 depicted by hatching is provided near the anode layer 104 of the high-resistance n-type drift layer 101.
FIG. 6 is a general chopper circuit diagram of an insulated-gate bipolar transistor (IGBT) and a diode. While there is a floating inductance Lstray in a closed circuit connecting the diode, the IGBT, and an intermediate capacitor, the floating inductance Lstray is depicted on a part of the circuit in FIG. 6 for the sake of convenience.
FIG. 7 is a reverse-recovery voltage and current waveform chart depicting temporal transitions of voltage and current at the time of switching of a general diode. FIG. 7 depicts reverse-recovery voltage and current waveforms representing transitions of reverse-recovery voltage and current with respect to the time (μs) in a case of turning off the diode operating on the circuit depicted in FIG. 6.
As depicted in FIG. 7, an anode current Iak decreases from a forward current If by a decrease rate di/dt to be commutated in the reverse direction and the reverse current further increases. After having reached a reverse-recovery peak current Irp, the anode current Iak decreases by a current decrease rate dIr/dt and converges to a current value 0. In FIG. 7, an anode-cathode voltage Vak is depicted in the direction of a cathode-anode voltage Vka in which the cathode is positive with respect to the anode, so as to be readily seen.
The anode-cathode voltage Vak changes from a forward voltage VF (not depicted) to a voltage in the reverse direction corresponding to the decrease of the anode current Iak, and the anode-cathode voltage Vak becomes negative (the cathode-anode voltage Vka becomes positive). When the anode current Iak then reaches the reverse-recovery peak current Irp, the cathode-anode voltage Vka has the same value as that of a power supply voltage Vcc. A voltage higher than the power supply voltage Vcc by a product (Lstray×dIr/dt) of the current decrease rate dIr/dt of the anode current Iak and the floating inductance Lstray is thereafter produced, becoming a surge voltage. When the absolute value of the current decrease rate dIr/dt becomes largest, the cathode-anode voltage Vka also has a largest value Vs of the surge voltage. The cathode-anode voltage Vka thereafter converges to the power supply voltage Vcc.
When the diode 100 is to be switched from a state in which the forward current (an anode current) is flowing to a state in which a reverse voltage is blocked at the time of switching the diode, the reverse current flows while the switching is completed, as depicted in the reverse-recovery current and voltage waveform chart of FIG. 7. This is because even when the direction of voltage application is reversed, carriers accumulated in the diode 100 by carrier conductivity modulation remain as residual carriers and become the reverse current during recombination and annihilation or discharge to the outside.
This reverse current is referred to as “recovery current (reverse recovery current)” of the diode. The peak value Irp of the reverse recovery current increases as the current decrease rate (dlr/dt) of the forward current increases. If the peak value Irp of the reverse recovery current becomes large, the switching loss becomes large. In the process of increasing the reverse recovery current, a depletion layer starts extending from the pn junction 106 a short time later and the reverse voltage (a blocking voltage) increases. The increased reverse voltage thereafter converges to a reverse-bias voltage value that is applied externally. Meanwhile, residual excess electrons in the n-type drift layer 101 are eliminated from the cathode electrode 103 through the cathode layer 105 and residual holes are eliminated from the anode electrode 102 through the anode layer 104. Here, the carrier mobility of the holes is lower than that of the electrons and, thus, the decrease rate dIr/dt of the reverse recovery current may be considered to depend on the elimination rate of the residual holes.
When the diode is switched from the forward current state to the reverse blocking-voltage state, the increase rate of the reverse voltage of the diode is larger as the current decrease rate is larger, which causes the electromagnetic noise described above. This is because the reverse voltage of the diode needs to be increased rapidly to eliminate the residual holes quickly in order to maintain the current decrease rate.
The reverse-recovery voltage and current waveform chart depicted in FIG. 7 can be roughly divided into two regions with respect to the time axis (μs) of the horizontal axis. One of the regions is a region A from when the forward current reaches zero until the peak value Irp of the reverse recovery current is reached. The forward current decreases from a steady current by the current decrease rate di/dt, which is determined by the drive frequency of the IGBT, or the like.
During this decrease, the current flowing when the holes remaining in the n-type drift layer 101 are eliminated from the anode electrode 102 is the reverse recovery current. The reverse recovery current increases with an increase of the reverse bias voltage and reaches the peak value Irp of the reverse recovery current. The other region is a region B from the peak value Irp of the reverse recovery current until when the reverse current becomes zero due to elimination of the residual holes from the anode electrode 102 and recombination thereof by the decrease rate (dIr/dt).
The reduction of the switching loss and the soft recovery characteristics demanded for a freewheeling diode have a tradeoff relation and therefore, normally, are not easy achieved concurrently. For example, the reduction of the switching loss is achieved by decreasing the amount of holes injected from the anode layer 104 to decrease the peak value Irp of the reverse recovery current and by increasing the current decrease rate dIr/dt to shorten a reverse recovery time (trr). However, the soft recovery characteristics are achieved by inversely decreasing the reverse-recovery current decrease rate dIr/dt in the region B to lengthen the reverse recovery time (trr). Because the measures for achieving the reduction of the switching loss and the soft recovery characteristics are thus conflicting, both reduced switching loss and soft recovery characteristics are not easily achieved.
To reduce the switching loss at the time of reverse recovery, a method of thinning the n-type high-resistance drift layer within a range in which the breakdown voltage of the device is not lowered, to thereby reduce the residual carriers (holes) is also conventionally used. However, in this case, carries accumulated on the cathode side at the time of reverse recovery are also reduced and thus, the residual carriers on the cathode side annihilate faster (the decrease rate dIr/dt of the reverse recovery current becomes larger). As a result, the surge voltage is increased and oscillation is likely to occur. That is, hard recovery characteristics are likely to result when the decrease rate dIr/dt of the reverse recovery current is large, and the loss becomes large when the decrease rate dIr/dt of the reverse recovery current is too small. Accordingly, it is normally quite difficult to reduce the switching loss while maintaining the soft recovery characteristics.
As described above, achieving both the reduction of the switching loss and the soft recovery characteristics (low noise) requires not only reduction of the amount of holes injected from the anode layer to decrease the peak value Irp of the reverse recovery current but also appropriate control of the duration of life (lifetime) of the injected holes.
For example, a method of forming a region having a short carrier lifetime within a desired depth range in a thickness direction of a Si semiconductor substrate in order to effectively control the residual carriers (holes) is conventionally known. As such a carrier lifetime control method, there is a method in which crystal defects formed by applying or introducing radial rays to a semiconductor are used as carrier recombination centers. While most of the crystal defects are recovered by thermal treatment at 200° C. to 400° C., complex defects associated with oxygen remain. A method of controlling the lifetime to a desired value by controlling the complex defects has conventionally been developed.
A method of thermally diffusing a heavy metal such as platinum into a semiconductor is also conventionally known. This method uses an impurity level formed in a Si band gap by crystal defects that are formed in a semiconductor substrate for carrier lifetime control. However, the carrier lifetime control method using a heavy metal is likely to cause segregation in crystal defects on a Si/oxide-film interface or in crystal defects in a highly-doped region. Therefore, while a region in which the minority carrier lifetime is short can be formed at these locations, a region having a short carrier lifetime is difficult to form at an arbitrary location.
Types of radial rays used for the lifetime control include helium irradiation, proton irradiation, electron beam irradiation, and the like. Among these types, the helium irradiation and the proton irradiation are short in the range in a semiconductor and thus can locally form a region in which the lifetime is controlled to be short and fall within a predetermined depth range. Meanwhile, a high-energy radiation apparatus is quite expensive and is not so high in the practical utility in view of the depth control accuracy when the thickness of a metal masking shield is used for depth control of the irradiation range.
While electron beam irradiation is superior in cost and productivity, the carrier lifetime becomes uniform throughout the semiconductor substrate in the thickness direction because the range in a semiconductor is long. Accordingly, local formation of a carrier lifetime region is difficult. However, after a high-concentration oxygen region is locally formed in advance in a semiconductor substrate, a part of the semiconductor region other than the high-concentration oxygen region is irradiated with an electron beam to such an extent that crystal defects effective for the carrier lifetime control are not formed. With this process, local carrier lifetime control can be provided to some extent by electron beam irradiation (see Japanese Laid-Open Patent Publication No. 2007-266103 listed below, for example).
Other documents related to the reduction of switching loss and soft recovering are described in the following. In particular, for example, one document describes that loss at the time of reverse recovery is reduced to suppress extension of a depletion layer by provision of a carrier capture layer near an intermediate region of a high-resistance region (Japanese Laid-Open Patent Publication No. 2010-92991). Another document describes, for example, that oxygen is introduced and protons are irradiated from an anode-side surface to introduce crystal defects and the crystal defects are recovered to increase a net doping concentration, thereby achieving low loss and soft recovery characteristics (see Domestic Re-publication of PCT International Application, Publication No. 2007-55352 listed below, for example). Another document describes, for example, that platinum is diffused in a high-resistance n-layer and the n-layer is irradiated with helium ions to form a low carrier lifetime region, thereby achieving soft recovering (see International Publication No. 99/09600 listed below, for example).